In one of our bathrooms, at some point a plumber installed smaller 3/8 O.D. copper pipe that come off of the 1/2 copper pipe from wall. Im guessing he added this to make a new connection that compression angle stops could screw onto in the future ( instead of having to have them soldered on.) Anyway we want to replace these in the bathroom, but something about the size we need for just this bathroom is confusing to me, and here is why. All other angle stop valves in our house are the standard 1/2 in NOM Compression Inlet x 3/8 O.D. outlet. But when I measure the actual copper inlet pipe for the 1/2 size valve for all of those it actually measures 5/8 O.D. SO why we needed 1/2 for all the others was odd to me. That just makes understanding what size we need for the one bath with smaller pipe more confusing. Now that this one bathroom has a even smaller than normal copper pipe 3/8 O.D. connected to our angle stop, a lady from Brasscraft told us we need a 3/8 Nom compression inlet x 3/8 outlet. why is the inlet size she said we needed the exact same as the O.D. of the copper pipe I measured for this one, when all the (1/2 inch inle)t for a pipe that really measured 5/8. I also enclosed a photo. SO do we need 3/8 x3/8 like she said Or does that 3/8 O.D really mean it is a 1/4 size fitting.